Medical devices capable of being implanted in the body to provide therapy to a recipient have become increasingly common over recent times. Devices such as pacemakers, defibrillators, cochlear implants and functional electrical stimulation systems have all proven successful in providing useful therapy to recipients across a broad spectrum of applications.
Fundamental to all such devices is the provision of an implantable stimulator unit fixedly implanted within the body of the recipient. This stimulator unit is typically capable of receiving control signals from a device external to the recipient via a transcutaneous link. As well as control signals, the implanted stimulator unit may also receive power from an external device via the same or an alternative transcutaneous link.
Upon receipt of control signals and/or power, the stimulator unit typically then directs and controls the stimulation to be applied by the system. In the case of cochlear implants, the stimulator system may select the desired electrode and send a stimulation pulse to the electrode having a desired amplitude and pulse width. Typically, the stimulator unit is provided with dedicated electronics which enable it to decode the received control signals and control the flow of stimulation current from the stimulator unit to the desired stimulation site.
With advancement in battery technology, it is becoming increasingly popular for implanted stimulator units to be provided with their own power source, usually in the form of a rechargeable battery, to provide operating power to the electronics of the stimulator unit. In this regard, such devices can operate, at least for a period of time without the need for any external devices. This is important for pacemaker devices as they do not need to rely upon a constant link with an external device to remain operational, and can continue to perform their important function by relying on their own power source. For devices such as cochlear implants, there is an increasing desire for such devices to operate invisibly without the need for external devices and for this reason the use of an implantable stimulator unit with its own power source is becoming increasingly desirable.
Apart from the implanted stimulator unit which houses the electronic circuitry and power source necessary to control the therapy applied by the implantable device, a means for actually applying the therapy is also fundamental to such systems. In most cases, the means for applying the therapy is typically one or more electrodes, strategically positioned close to the desired stimulation site, for applying the electrical stimulation to that particular site.
The stimulating electrodes are typically positioned remote from the implanted stimulator unit. For example, in cochlear implant applications the stimulator unit is typically positioned in a recess in the skull whilst the electrodes are implanted in the cochlea close to the desired nerves. In this regard, a lead connecting the electrodes and the stimulator unit is required, and such leads need to be designed in a manner to ensure that the electrical stimulation is delivered safely to the appropriate electrodes and that the link between the stimulating electrodes and the stimulator unit is sturdy and reliable.
Traditionally, the common way of providing this electrical connection between the stimulator unit and the electrodes has been via conducting wires within the lead. Such wires typically communicate with the electronics within the stimulator unit via a hermetic feedthrough device and are welded to the terminating electrodes thereby forming a conductive path from the stimulator unit to the electrodes along which the stimulation current can flow. Typically, the lead is insulated from the surrounding tissue via a coating of insulative material, such as silicone.
In providing such an implantable connecting lead, it is important that the lead is capable of a degree of flexibility to compensate for any movement between the implantable stimulator and the electrodes, such as movement which may naturally occur due to body growth. Without such flexibility, excessive force can be experienced in the lead, particularly at the connection points such as at the feedthrough, resulting in the lead failing to act as a conductor. Further to this, providing a flexible rather than a rigid connection between the electrodes and the stimulator unit provides for easier surgical placement of the electrodes close to the desired stimulation site, which ensures that the surgical procedure is simpler and requires less surgical skill.
The typical method of providing a lead capable of a degree of flexibility is to dispose the wires, either individually or as a group, in a helical arrangement along the length of the lead. The wires can then be enclosed in a coating of body-compatible polyurethane, or a suitable nonconductive plastic which has a requisite degree of flexibility. In this way, the lead can experience a degree of elongation without placing undue stress on the wires or at the point where the wires connect to the stimulator unit. Examples of such leads are described in U.S. Pat. No. 4,835,853 and International Patent Application Publication No WO 83/04182.
One problem with such prior art methods is that it is difficult to sort the wires in a manner that makes it easily identifiable which electrode they are connected to. As such, following the formation of the lead, it is a time consuming process to individually test each wire and identify which electrode it is connected to and to then ensure that this wire is connected to the stimulator unit in the appropriate manner. This problem is further exacerbated when the number of stimulating electrodes increases and hence the number of wires increases, such as in cochlear implants where the number of electrodes can be greater than 22.
The present applicant has developed a new process for manufacturing electrodes and conductors that connect the electrodes to a stimulator/control unit. This process and the resulting products are described in detail in International Patent Application No. PCT/AU02/00575, the contents of which are incorporated herein by reference. In essence, this process results in the formation of an electrode array comprising of a stack of offset electrodes, layered on top of each other. Each of the electrodes has a respective conducting portion extending from the electrode, with the conducting portion and the electrode being integral and constructed from one piece of material. In this regard, a connecting lead is provided consisting of a plurality of layered, parallel conducting portions extending in a longitudinal direction. Such a lead therefore resembles a layered ribbon conductor, considerably different from conventional wire leads.
With such a change in the traditional structure of conventional wire conductors used in implantable devices, there is a need to provide a conducting lead that is capable of maintaining the conductors in a flexible and insulative environment. Further to this, there is a need to provide a conducting lead that can take advantage of the ordered structure of layered conducting wires so that the conductors can be easily sorted and connected to the appropriate stimulator.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.